As a player and elite donator of SPK, you can tell I obviously love the game. But there's a few things that could potentially give the average player more to do and more ambition to go for a skilled account instead of pking account...
Fletching:The first suggestion that was on my mind was to do with fletching. the only type of bolts that I know of that you can use is dragon (excluding elementals). As incentive for leveling fletching, for PvMers the different bolts could be added (ruby bolts (e) for the extra damage output) and possibly onyx bolts? These both only being obtainable via fletching.
Crafting: This plays into crafting as well, as the gems could only be obtainable through mining/killing monsters that had the drop rate for onyx/ruby/etc. Cut the gems, craft the bolts.
Herblore: We have items obtainable such as super combat pots, but they're not craftable. I don't have an issue with this, but perhaps overloads could be added as a herb option to give an incentive to level herblore, or be able to craft super combats but leave overloads as whatever.
Fishing: Level fishing to obtain higher tier fish for normal players, like donators can spawn rocktails but if a normal player is ambitious enough to level his/her fishing and fish out the rocktails, more power to them. (If this is a thing already call me stupid in the comments)
The last thing that crossed my mind was how we have dzone and ezone etc, why not add a small area for people how have the completionist/maxxed cape to chill at? Just to give them a little more of a reward for sticking the time into the grind.
If you made it this far, thank you for reading my suggestions. Thanks!